1. Field of the Invention
The present invention relates to a constant-current drive circuit which causes a light emitting device such as LED (Light Emitting Diode) to emit light with target brightness.
2. Description of the Background Art
In order to utilize LEDs having RBG colors to display various colors, it is necessary that LEDs having the RGB colors stably emit the light with the similar brightness. In addition to a back light used in electronic instruments such as a portable telephone, the color display with LED comes into widespread use among various fields such as illumination for a speed meter of an automobile and a display device. There are various techniques of driving LED at constant current for the purpose of the stable light emission of LED. For example, Japanese Patent Application Laid-Open No. 2000-214825 proposes a technique of performing PWM (Pulse Width Modulation) control to stably drive LED having RGB colors.
In Japanese Patent Application Laid-Open No. 2000-214825, a constant voltage is applied to each of LEDS having the RGB colors, and a difference of forward drop voltage is adjusted in each LED by an adjusting resistor provided in each LED, which enables the stable light emission at constant current. The technique proposed in Japanese Patent Application Laid-Open No. 2000-214825 can sufficiently serve a relatively-small-scale circuit such as the portable telephone. However, in a large-scale circuit such as a large-scale display device in which high brightness is required by utilizing many LEDs as the back light, it is difficult that a fluctuation in forward voltage is individually adjusted by the adjusting resistor. Therefore, the brightness of LED cannot be kept uniform by the constant-current drive, which results in a problem that degradation of image quality such as unevenness of brightness is generated in the display.
In Japanese Patent Application Laid-Open No. 2000-214825, a drain of a switching FET is connected onto a cathode side of LED, a voltage generated by a D/D converter is applied to an anode side of LED, and a control unit applies a pulse voltage to a gate of the switching FET by PWM (Pulse Width Modulation) control to control a switching operation, which allows LED to be driven. That is, on-and off control of the pulse voltage is performed to control the lighting of LED by PWM. At this point, the control unit also controls the D/D converter.
However, in the method proposed in Japanese Patent Application Laid-Open No. 2000-214825, sometimes there is generated a problem when LED is lit on. Specifically, in lighting LED on from a state, in which LED is turned off while the voltage is not applied, even if the control unit controls to turn on the switching FET at the same time as start of the application of the constant voltage to LED, because the generation and application of the voltage is started at that moment by the D/D converter, the sufficient voltage is not obtained, or the voltage generated by the D/D converter is incorrectly controlled by an influence of a charge remaining between the control unit and the D/D converter. Therefore, from the viewpoint of waveform of the current passed through LED, although the control is performed such that the pulse waveform rises steeply from the turn-off to the turn-on of LED, actually a time lag is generated until the intended current is obtained, and sometimes rounding in which the current is gradually increased is generated in the current waveform. In this case, LED cannot emit the light with the originally intended brightness until a predetermined current is obtained. The generation of the phenomenon does not become problematic, in the case of the relatively-small-scale circuit such as the portable telephone in which each one of LEDs having the RGB colors is utilized, or in the case where LED is utilized in the instrument in which high-speed operation is not required. However, the drawback that the brightness is changed according to the rise of the current becomes prominent, in the electronic instrument such as the display which is realized by connecting many LEDs in series or by utilizing a high-current drive type of high-brightness LED. Therefore, there is a problem that this method cannot be utilized for the display in which the high-speed operation is required, e.g., a moving image is displayed.
The phenomenon can be avoided when the current voltage is continuously applied to LED even if LED is turned off by the switching FET. However, in this case, an excessive inrush current is passed through LED once the switching FET is turned on, which results in a problem that components such as LED which constitute the circuit is possibly broken.